Mary's got something that the other two major female characters don't have: common sense. She's as intelligent as Dorothea, but without Dorothea's unrealistic ambitions. She's been as well educated as either Dorothea or Rosamond, but she doesn't have Rosamond's pretentious, artificial veneer. But Mary's hardly a traditional heroine. She's got a lot going for her in the smarts department, but her family isn't wealthy, and she's not all that pretty. In fact, we're told early on that she looks generic. When we say "generic," though, we don't mean, "boring." In fact, the narrator says that her "plainness" is universal and timeless:
Advancing womanhood had tempered her plainness, which was of a good human sort, such as the mothers of our race have very commonly worn in all latitudes under a more or less becoming headgear. Rembrandt would have painted her with pleasure, and would have made her broad features look out of the canvas with intelligent honesty. For honesty, truth-telling fairness, was Mary's reigning virtue: she neither tried to create illusions, nor indulged in them for her own behoof, and when she was in a good mood she had humour enough in her to laugh at herself. (1.12.68)
In other words, Mary Garth looks like some kind of universal, every-woman. She's not pretty, but she resembles the everyday women that Rembrandt used to paint (see "Best of the Web" for an example). There's something timeless about her.
George Eliot is fond of toggling between the universal and the individual, and her descriptions of Mary Garth are no exception. Try reading through this passage, and notice how it moves from the general (all the women in the street) to the specific (the one that looks like Mary Garth):
If you want to know more particularly how Mary looked, ten to one you will see a face like hers in the crowded street to-morrow, if you are there on the watch: she will not be among those daughters of Zion who are haughty, and walk with stretched-out necks and wanton eyes, mincing as they go: let all those pass, and fix your eyes on some small plump brownish person of firm but quiet carriage, who looks about her, but does not suppose that anybody is looking at her. If she has a broad face and square brow, well-marked eyebrows and curly dark hair, a certain expression of amusement in her glance which her mouth keeps the secret of, and for the rest features entirely insignificant – take that ordinary but not disagreeable person for a portrait of Mary Garth. (4.40.67)
Mary doesn't mind being plain, but it can make her a little bitter, especially when she's standing next to someone as pretty as Rosamond. During their first meeting together, she says, "What a brown patch I am by the side of you, Rosy! You are the most unbecoming companion" (1.12.30). Mary's chief fault is that she can be bitter towards people like Rosamond who can't see past her plainness and don't appreciate her good sense. On the other hand, though, she doesn't hold grudges and "if you did her a kindness, she would never forget it" (4.40.67).